Outside groups have poured millions of dollars into North Carolina’s Senate race, more money than in any other state, but who’s behind a lot of it remains secret.

And much more is expected.

Supporters of transparency in government say all this matters because the people watching political ads on TV often don’t know who’s paying for them and why.

Outside money in campaigns isn’t new, nor is the puzzle of who’s behind it. But in 2010, a Supreme Court ruling in the case of Citizens United v. Federal Election Commission overturned a ban on corporate and union involvement in federal elections and allowed special interest policy groups across the political spectrum to spend unlimited amounts on independent political ads _ often without having to disclose their donors.

The ruling opened the floodgates on campaign spending, and this election year North Carolina seeing a lot of the fallout because it’s a key battleground for control of the U.S. Senate in 2015. Republicans need to pick up six seats to take over the majority, and they see first-term Democratic Sen. Kay Hagan as vulnerable. Their nominee will be decided in the May 6 GOP primary.

Big political spending and lots of television ads are not new to the Tar Heel State.

“This is a state that’s been a test kitchen for assault-your-opponent ads,” said Ferrel Guillory, director of the Program on Public Life at the University of North Carolina at Chapel Hill and a long-time observer of statewide politics.

In 1984, the late Republican Sen. Jesse Helms ran the most expensive Senate race in American history at the time, he said.

Fast-forward to this past week, and the same U.S. Supreme Court majority that opened the taps for unlimited corporate spending with the Citizens United decision went even further in a ruling that lifts the aggregate contribution limits for individuals. Donors will now be able to give the maximum amount allowed to as many candidates and state political parties as they want.

While many Republicans and conservatives cheered the ruling as a victory for the First Amendment, Rep. David Price, a Democrat from Chapel Hill and former political science professor at Duke University, said the high court's conservative majority was "systematically destroying...laws that limit the influence of big money in politics."

"Increasingly, a few mega-donors are drowning out the voices of small contributors and ordinary citizens,” he said. “This compromises trust in government and puts the integrity of our elections at risk."

The most recent big injection of political spending in North Carolina came on April 1 from American Crossroads, a super PAC created by Republican operative Karl Rove. The group spent $1.1 million for ads for state House Speaker Thom Tillis, its preferred candidate in the Republican Senate primary.

The ad says Tillis is “true to our values” and lists his support for tax cuts and the state’s more restrictive voter ID rules, as well as his opposition to the Affordable Care Act. It will run for four weeks, nearly up to the Republican Senate primary.

The American Crossroads ad follows $8.3 million spent on six ads against Hagan and the health care law, which she supported, by Americans for Prosperity, a group backed by billionaire industrialists Charles and David Koch. Koch Industries, Inc., is headquartered in Kansas.

The campaigns, which have trackers checking ad buys, report that outside groups have spent about $10 million against Hagan or for her challengers. On the other side, groups have spent more than $5 million on her behalf or against Tillis.

Hagan’s largest single source of outside support has been $2.4 million from the Senate Majority PAC, a super PAC founded by Democratic strategists who want their party to hold onto control of the Senate. It has spent more on ads for Hagan so far than for any other Democratic Senate candidate.

Former New York Mayor Michael Bloomberg, who gave the PAC $2.5 million last year, has been its largest donor so far this cycle. The Massachusetts Teachers Association, a union, has given $700,000.

A Senate Majority PAC ad says the “out-of-state” Koch brothers back Tillis and that together they would be “a win for the special interests.”

Other large Hagan ad buys have been by Democratic Patriot Majority USA, which has spent more than $800,000. Its donors remain veiled.

In addition, at least three ads about reducing air pollution included in the Republican media tracking commend the senator, but don’t specifically ask people to vote for her. They were purchased by the Southern Alliance for Clean Energy, the Environmental Defense Fund and the League of Conservation Voters. Total spending by the three groups was about $1.6 million.

The Council for American Job Growth paid $216,000 for an ad that thanked Hagan for supporting policies related to high-tech jobs. The group is part of Fwd.us, which is backed by tech industry leaders, including Facebook co-founder Mark Zuckerberg and Microsoft co-founder Bill Gates. Fwd.us wants immigration policy changes, including a path to citizenship for people without legal residency.

In a campaign video, Hagan said she needed support to “fight back against the outsiders like the Koch brothers and Karl Rove who have made me their number one target.”

Tillis spokesman Jordan Shaw said that Hagan was being hypocritical because her campaign has also received outside support.

“Kay Hagan and her far-left liberal allies are spending millions attacking Thom Tillis because they know he will end the train wreck in Washington that they have created,” he said.

The outside groups behind the Republicans in the race include FreedomWorks for America, a conservative super PAC that partially discloses donors. It has spent more than $78,000 on political messages against Hagan and for Republican Senate Republican primary candidate Greg Brannon of Cary, according to tracking by the Center for Responsive Politics, a nonpartisan campaign watchdog group.

But the biggest buy by far has been the six ads from Americans for Prosperity against Hagan and the health care law, which Republicans have been trying to repeal for several years.

“The reason we’re spending the money now is it’s an important issue to North Carolinians, and with an election going on the issue is being hotly debated,” said AFP spokesman Levi Russell.

The Hagan campaign, however, claims the attacks by the Koch-affiliated group go beyond the health care law and indicate a “Tillis-Koch agenda.” The campaign cites Tillis’ record in the state House and his opposition to a Democrats-backed federal minimum wage increase to $10.10.

What impact the influx of outside spending will have on the Senate contest is hard to say.

“I would say at least so far it’s been pretty limited,” said David Rohde, a political science professor at Duke University who has followed campaign finance throughout his career.

What’s immediately worrisome, he said, is the public’s perception that those who give a lot of money get their way. As a result, “faith in the system itself is undermined.”